Coatings for use on reflectors of electric lamp assemblies are well known in the art. Early coatings included metallic elements that were expensive, such as gold or platinum, or elements that were or susceptible to oxidation, such as silver or aluminum. These coatings reflected both visible light and infrared radiation, but had significant disadvantages. The coatings utilizing expensive elements were economically impractical for many uses, while the oxidation-susceptible coatings were short-lived due to a relatively rapid decrease in their effectiveness because of oxidation.
Dichroic coatings were developed, in part, as inexpensive and oxidation-resistant alternatives to metallic coatings. Dichroic coatings include layers of oxides of materials that typically have low absorption of radiation in the visible light and infrared portions of the electromagnetic spectrum and that are thermally and mechanically stable. Examples include oxides of titanium, silicon, tantalum, magnesium, aluminum, niobium, hafnium, cerium, zirconium, yttrium, erbium, europium, gadolinium, indium, bismuth, thorium and other suitable rare earth metals.
Dichroic coatings of the prior art either reflect radiation within the visible light portion of the electromagnetic spectrum while transmitting radiation in the infrared portion of the spectrum, or transmit visible light while reflecting infrared radiation. These coatings of the prior art have not exhibited a high level of reflectance of substantially all wavelengths in the both the visible spectrum and a portion of the infrared spectrum.
For example, dichroic coatings that transmit visible light and reflect infrared radiation have been used on the glass tube encasing a lamp filament. The coating allows light produced by the filament to pass through the glass but reflects infrared radiation and the heat associated with such radiation back to the filament. This reflection of infrared radiation and its associated heat helps to maintain the elevated operating temperature of the filament, thereby decreasing the amount of energy required by the filament to maintain a temperature of incandescence.
Dichroic coatings that reflect visible light and transmit infrared radiation are often used on lamp reflectors. A reflector having this type of dichroic coating reflects the visible light produced by the lamp, typically in one general direction so as to increase the illuminating efficiency of the lamp in that direction. However, the dichroic coating on the reflector allows infrared radiation and its associated heat to pass through the reflector. Transmittance of infrared radiation and its heat results in an increased heat load on the components of the lamp, such as seals, foils, ballasts, fixtures, and transformers, as well as components of other items near the lamp, such as polymeric housings.
The continued exposure to infrared radiation degrades many of these components as a result of the radiation alone. In addition, the increased heat load created by the infrared radiation results in greater than normal thermal degradation. Further, the increased heat load also tends to increase oxidation of surrounding components, creating a shorter life span for those components.
As a result, it is desirable to develop coatings which have the ability to reflect both visible light and a substantial portion of the infrared spectrum at a significant level, while having the advantages of low cost and oxidation resistance possessed by dichroic coatings.
In an exemplary embodiment of the present invention, an interference coating for reflecting both visible light and infrared radiation is provided. The coating includes a dichroic structure of a plurality of layers of a material having a low index of refraction and a plurality of layers of a material having a high index of refraction. The coating has an average spectral high reflectance of at least 80% for wavelengths in the visible light section of the electromagnetic spectrum and of at least 50% for wavelengths in a portion of the infrared section of the electromagnetic spectrum at least 150 nm wide.
In another exemplary embodiment of the present invention, an interference coating for reflecting both visible light and infrared radiation is provided. The coating includes a dichroic structure including a plurality of alternating layers of an oxide of silicon and an oxide selected from the group consisting of titanium, tantalum, niobium, hafnium and combinations thereof. The coating has an average spectral high reflectance of at least 80% for wavelengths of from about 400 nm to about 800 nm and of at least 50% for wavelengths in a portion at least 150 nm wide of the electromagnetic spectrum beyond a wavelength of about 800 nm at a normal angle of incidence.
In yet another exemplary embodiment of the present invention, an electric lamp including a reflector for reflecting both visible light and infrared radiation is provided. The reflector includes a substrate and at least a portion of the substrate is coated with a dichroic structure including a plurality of layers. The coating has an average spectral high reflectance of at least 80% for wavelengths in the visible light section of the electromagnetic spectrum and of at least 50% for wavelengths in a portion of the infrared section of the electromagnetic spectrum at least 150 nm wide.